William in his scarlet dress uniform as an honorary colonel of the Irish Guards; Harry in his Blues And Royals blue uniform with the earned rank of captain.  I'm guessing Charlie will wear his Royal Navy uni with earned rank of admiral.  The women get to wear hats. 

The Abbey verger is the fellow in the scarlet robes at the entrance.  Most parishes once had the role of verger; nowadays only very large parishes and cathedrals do, especially as its a paid position.